This invention relates to a method and apparatus for continuity testing of paths in a time division multiplex switching network.
Communication switching systems require switching networks to selectively interconnect lines to establish paths for calls. There are many types of known switching networks. Most of these switching networks use metallic contacts to close paths between an input terminal and an output terminal. In systems using marker control of crosspoint or crossbar switching networks, it is known for the marker to apply a given potential to one terminal and to test at the other terminal for the presence of this potential, to thereby verify that a continuous path through the network exists for the call.
With the advent of transistors and other solid state switching devices, efforts have been made to provide electronic switching systems using such devices both for control purposes and as the actual switching devices in the network. In time division multiplex switching networks, the signals at incoming terminals of the network are periodically sampled during recurring frames, each terminal being assigned as interval of the frame designated a time slot during which its signal is sampled and transmitted through the switching network to an output terminal. The time slots are thus intervals for carrying signals within each frame. Usually corresponding time slots in each successive frame carry successive portions of a signal; for example, a third time slot of each frame carries successive portions of one signal.
Time division multiplexing principles have also been applied to the transmission of communication signals. One specific application of this principle is known as the T1 carrier system. In that system there are provided 24 channels in corresponding time slots, and the signal is pulse code modulated, providing eight bits which are transmitted for each channel in its time slot during each frame.
Systems are now being developed for switching of the channels from T1 carrier and similar systems directly in the pulse code modulated digital form. If the signals in individual time slots on one line are to be switched to a different line through the network, the lines must be connected only when the time slot carrying a signal for a desired one of the other lines arise at a device connecting the lines. But one example of the many such networks is a network of highways in a time-space-time or TST telephone system.
In a known TST telephone system, call signals from several sources are time division multiplexed into time slots on transmission lines. Several of these multiplex lines are coupled to a time slot interchanger. Each interchanger serves as an input device for one of several input highways of the TST network. Each time slot interchanger has a buffer memory device for receiving the signals from each connected multiplex line and a control memory device for assigning the signals to time slots on the input highway.
Crosspoints connect each input highway to each of several similar output highways. Each output highway then carries signals in time slots synchronized with time slots on the input highway through a highway interchanger. These time slot interchangers serve as output devices for directing the call signals to discrete time slots on multiplex output lines to ultimately reach the call destinations.
To maintain call signal integrity each time slot carries the signal of only one call. It is therefore necessary to select an idle time slot on the input and output highways for each call signal to be added to the highways. The selection of the idle time slot is called an idle path search. A method and apparatus for such an idle path search in a time division multiplexed switching network is disclosed in my application, Ser. No. 428,791 now U.S. Pat. No. 3,912,871 filed Dec. 27, 1973.
The testing of continuity of the call path in a switching network is an important part of system maintenance. A more desirable feature would be to have the continuity test a part of every call processed. This means that prior to establishing a call, the call path to be placed in service is tested for continuity. Such continuity testing is known for space division switching networks, particularly for those having metallic contacts. However, for time division multiplex switching networks, a general procedure is to test the control memories and to assume that the paths are not faulty. In general, path continuity tests are part of a periodic maintenance routine rather than a part of normal call processing. A complete time division network path for a call generally comprises two parts for the two directions of transmission, and both parts should be tested.